HOW TO CHOOSE AN ADULT RESCUE DOG
A rescue dog doesn't have to be a mongrel. Most shelters have pure bred dogs of the main breeds as well as mongrels though you may have to wait for one to turn up. Or get a pedigree from the breed rescue organisation -- Kennel Club will give details of your nearest branch - www.the-kennel-club.org.uk  Remember that most pedigree dogs in rescue shelters won't have their pedigree papers so, although you will get the looks, you won't be able to show them.
Any good rescue establishment should come to check your home. If it doesn't do home checks, don't get a dog from them. A good rescue will try to match human and dog. Listen to their advice. If you are aged 80 with an arthritic hip you do NOT want a Jack Russell or a Dalmation! There are some quite shocking rescuers around. If you go  to one and find the place teeming with dogs, cats and heaven knows what else in bad conditions, report them to the RSPCA.

WHAT TO LOOK FOR AT THE SHELTER
Better shelters will show you pictures and then bring out a dog or two they think would suit you, so that you can meet them outside the kennel. These are the best shelters because they are making sure you don't just choose on looks. And they are sparing their dogs the stress of being always looked at by a stream of people.
If the shelter is the less modern kind with walk-through kennelling, walk right through the kennels before you make your choice. Look at them all. Some people get stressed by the barking and just choose the nearest likely one. Don't ignore that brown or black undistinguished mongrel which is wagging its tail. It may not look good but it will be loyal and true. You could be missing the best of all. Quiet dogs have a lot to give but are often overlooked.
Do not let pity get the better of you. Pity is a bad basis for a lifelong relationship. In any dog shelter there will be frightened and unhappy dogs, which are going to need rehabilitating. If you just think love will solve its problems - DON'T do it. Love is not enough. If you take on a dog with problems, the poor dog will simply end up back in the shelter and might end up being euthanised. Difficult dogs need expert owners.
Choose for temperament and behaviour rather than shape or colour. Dogs that offer a paw should be OK. Dogs that come forward, tail wagging and ears forward are OK. But watch where the ears are - ears back means fear even if the tail is wagging. Dogs that cower at the back of the run or come forward and bark at you, then run back in fear, should be avoided ubnless you are an expert.
If the kennel person tells you not to put your fingers through the wire, then it bites!

WILL IT FIT YOUR LIFESTYLE?
Always check if a dog is used to cats, if you have cats. If the shelter doesn't know about this, go to another shelter that does. Your cat's happiness matters too. Don't just chuck a dog into the cat's territory and expect it to manage. It will probably leave home or it may even be injured if the new dog is a big breed.
Do you have children? Does the rescue centre know if the dog likes children? If the centre doesn't know, this is not a dog for a family with young children.
Does the dog like other dogs? Ask them if they will take it out with other dogs to see what happens. If it doesn't like other dogs, it may be difficult to walk in the park.
Will your new dog like your existing dog? Take it for a walk with your existing dog to see how they get on. If you have a male, add a female or visa versa. Most difficult are two females.
Ask the rescue people if the dog can be left alone. If they don't know, discuss this possibility with them. Sometimes it is only a temporary problem. But some dogs have severe separation anxiety and you will need pet behaviour counselling to get it right. Will the rescue shelter give you behavioural advice or pay for it, and support your rehab efforts?
Think about exercise and how much time you have. The younger the dog the more it needs. Border collies, Jack Russells and other terriers need a lot to do -- games as well as exercise. Avoid working dogs unless you can offer two hours exercise every single day.
Elderly dogs are ideal for elderly people or people who cannot walks for miles. Occasionally rescue shelters will pay bills for elderly dogs.
Blue Cross (www.bluecross.org.uk ) and the Dogs Trust (www.dogstrust.org.uk) , Wood Green, and Battersea (www.dogshome.org) are best organised and most have a behaviour adviser for any problems. They also have good literature. You can go back to them for further advice. Among other rescues, rescues that specialise in dogs are often better than rescues that take all pets. RSPCA centres vary from the excellent to the not so good.